i
CO
2
capture with liquid crystals:
a phase equilibrium study
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus Prof. Ir. K.Ch.A.M. Luyben; voorzitter van het College voor Promoties,
in het openbaar te verdedigen op dinsdag, 26 mei, 2015 om 15:00 uur
door
Mariëtte DE GROEN
ii
Dit proefschrift is goedgekeurd door de promotor: Prof. Dr. Ir. T.J.H. Vlugt copromotor: Dr. Ir. Th. W. de Loos
Samenstelling promotiecommissie bestaat uit: Rector magnificus, voorzitter
Prof. Dr. Ir. T.J.H. Vlugt, promotor Dr. Ir. Th.W. de Loos, copromotor Prof. Dr. Ir. A.B. de Haan, TNW, TU Delft Prof. Dr. S.J. Picken, TNW, TU Delft Prof. Dr. U.K. Deiters, Universität zu Köln
Prof. Dr. E.J. Meijer, Universiteit van Amsterdam Dr. Ir. B. Breure, Shell
This research is supported by the Dutch Technology Foundation STW, applied science division of NWO and the Technology Program of the Ministry of Economic Affairs.
iii
Table of Contents
1. Introduction to CO2 capture ... 1
2. Phase theory of binary mixtures of a liquid crystal and a gas ... 5
3. Phase behaviour of liquid crystals with CO2 ... 11
4. Phase behaviour of the system 4’-pentyloxy-4-cyanobiphenyl + CO2 ... 23
5. Henry coefficients of selected binary mixtures of a liquid crystal + CO2 ... 37
6. Binary and ternary mixtures of liquid crystals with CO2 ... 47
7. Phase behaviour of binary mixtures of a liquid crystal and methane ... 67
8. Concluding remarks ... 75 References ... 77 Summary ... 85 Samenvatting ... 89 Curriculum Vitae ... 93 Published work ... 95 Acknowledgement... 97
Appendix A. Experimental data of Chapter 3 ... 99
Appendix B. Experimental data of Chapter 4 ... 105
Appendix C. Experimental data of Chapter 5 ... 113
Appendix D. Experimental data of Chapter 6 ... 117
Appendix E. Experimental data of Chapter 7 ... 131
1
1. Introduction to CO
2capture
In recent years, research on global warming remained an important issue, especially research to prevent the exhaust of greenhouse gases [1-6].The main goal of several climate initiatives is to lower the atmospheric concentration of greenhouse gases [1,3,7]. As the production capacity of green energy is not developing fast enough to substitute fossil fuels, capturing CO2 is needed to
control the atmospheric CO2 concentration [7-9]. Several methods for capturing
CO2 are available on the short term, for example post-combustion CO2 capture
with amines, pre-combustion CO2 capture with solvents like Selexol or Rectisol,
or the oxyfuel process [10-16]. However, all capturing methods increase the energy consumption of the fossil fuel power plant. In the case of amine absorption, the energy consumption lowers the production of the complete power plant by 30% to 40% [2,17].
Most commercial processes involve post-combustion CO2 capture from flue gas.
This method is already available since the 1930s, for example for the removal of CO2 from natural gas [18,19]. The main advantage of post-combustion CO2
capture is that it can be incorporated easily in existing power plants [17,20]. One of the most used commercial methods is alkanolamine scrubbing [10,19,20]. An aqueous mixure of an alkanolamine is brought into contact with a gaseous stream containing CO2. The CO2 binds chemically with the amine component, the
CO2 lean stream is released to the atmosphere. The amine complex is
regenerated in another column with steam. The main drawback of this process is that regeneration of the solvent is energetically intensive. Other drawbacks are emissions of alkanolamine, the formation of aerosols and corrosion of the equipment [21-24]. Therefore, an ongoing search is to find solvents needing a lower regeneration temperature [20]. Currently, molecules like diisopropyl-amine (DIPA) and tertiary diisopropyl-amines like methyldiethanoldiisopropyl-amine (MDEA) are preferentially used because of their lower regeneration temperature [8,24]. Other post-combustion processes which are currently investigated are, amongst others, membrane processes and processes based on the use of phase change ionic liquids, and various types of solid sorbents such as Metal Organic Frameworks (MOFs) or zeolites [24-27].
For new installations pre-combustion capture is considered as a viable option, though no process is found yet with small energy use. Conventional methods are the Selexol and Rectisol processes [28,29]. These processes are based on physical
2 CnH2n+1 N O N CnH2n+1 CnH2n+1 N CnH2n+1 CnH2n+1 O CnH2n+1 N N
Figure 1.1. Molecular structures of the liquid crystals used in this study.
dissolution of CO2 using alcohols at elevated pressure [28]. There is an ongoing
search for new materials for capturing CO2 in a more energy efficient way.
Some new materials for pre-combustion CO2 capture are, amongst others, metal
organic frameworks, zeolites and ionic liquids [24,26,28,30]. It is not clear yet which technology will eventually become most promising.
Recently, a process was proposed using liquid crystals for CO2 capture by Gross
and Jansens [31]. The key property of liquid crystals is the structured liquid phase. In case of a nematic phase this is a phase with similar viscosity as the isotropic phase, but the molecules have orientational ordering [32]. The shape of the molecules causes this: liquid crystals have a rigid core, consisting of two or more benzene and/or cyclohexane rings with side tails at the 4- and 4’-positions. Examples of the general structures of the liquid crystals used in this study are shown in Figure 1.1. The solubility of CO2 is lower in the nematic
crystalline phase than in the isotropic disordered phase, because of e.g. a free volume difference [33,34]. If the liquid crystal solvent is saturated in the isotropic phase, and is cooled down at constant pressure until the nematic phase is reached, CO2 is released from the solvent [31]. This potentially can lead to a
lower energy use for capturing CO2, because the solvent is regenerated by a
temperature change of a few degrees Kelvin only [31].
Two parameters are in general important for this process: the solubility of CO2 in
the liquid crystal solvent and the amount of CO2 released from the solvent in
each absorption/desorption cycle as described in the previous paragraph. Very little is known about the solubility of CO2 in liquid crystals [35,36]. Of these liquid
3 with a gravimetric method. The article of Chen describes the solubility of CO2 and
of different gases like N2 and He in liquid crystals [37]. From this study, it is clear
that CO2 is better soluble than the other gases.
Other types of phase behaviour of liquid crystal + CO2 systems, like the study of
three-phase curves, or the phase transitions from the nematic to the isotropic one-phase areas, are not studied in literature. The only articles on phase behaviour of liquid crystals with solutes published are of T,x-diagrams of mixtures of alkanes [38,39] or organic solutes [40,41] and liquid crystals. The research discussed in this thesis focuses on the experimental determination of P,T,x-phase diagrams of CO2 + liquid crystal systems. A short overview of phase
theory of these kind of systems is presented in Chapter 2. In Chapter 3 of this thesis binary phase diagrams of a liquid crystal with CO2 are discussed. Chapter
4 deals with a complete P,T,x-phase diagram of the binary system 5OCB + CO2. In
Chapter 5, phase diagrams of liquid crystals with CO2 at lower CO2 concentration
and the Henry coefficient of these systems are discussed. Chapter 6 discusses ternary phase diagrams of mixtures of two liquid crystals + CO2. Finally, Chapter
7 deals with the solubility of methane in various liquid crystals. An overview of the molecular structures of the liquid crystals used is presented in Appendix F. The main conclusion of this thesis is that liquid crystals of the phenyl-cyclohexyl type are most promising for a CO2 capture process with liquid crystals. Methane
is best soluble in apolar liquid crystals. Using binary mixtures of liquid crystals as a solvent leads to better solvent properties for CO2. However, currently the liquid
5
2. Phase theory of binary mixtures of a liquid crystal
and a gas
Phase theory is the section of thermodynamics describing phase equilibria of one or more substances. These phase equilibria are generally described as functions of pressure P, temperature T and composition x. The main strength of phase theory is the ability to describe a phase diagram in a qualitative way with a minimum of experimental data available [42].
To reach thermodynamic equilibrium, the chemical potentials µi of component i
should be equal in all the different phases:
α β π
...
i i i
(2.1)This equation is valid for n components in π phases. For all the lines and points described in Figure 2.1 this condition holds.
The number of degrees of freedom at equilibrium according to the phase rule of Gibbs is:
π 2
F n
(2.2) This equation tells us that the number of degrees of freedom F for any equilibrium in the diagram equals to the number of components n plus 2, with the number of phases π and the number of additional relations φ subtracted from it. As an example, for a system of two phases, i.e. liquid + vapor, in a binary system, the number of degrees of freedom is 2. When two variables are chosen, the system is completely described. For example, if pressure and temperature are chosen, the composition of the liquid and gas phase are fixed.2.1. Unary systems
Unary systems have, in general, three different kind of phases: a gas phase G, an isotropic liquid phase I and one or more solid phases denoted with Si. Liquid
crystals have, beside these S, I and G phases one or more liquid crystalline phases. A liquid crystalline phase can be, for example, a nematic phase N or a smectic phase Sm [32]. Smectic phases exist in various types, depending on the type of ordering. In the following, all these liquid crystalline phases are lumped
6
Figure 2.1. Schematic P,T-diagram of a pure liquid crystal. S denotes a solid phase, C the liquid crystalline phase, I the isotropic phase and G the gas phase. denotes the triple points of the liquid crystal.
in one phase, called a condensed phase with symbol C. Figure 2.1 shows a schematic unary phase diagram of such a generalized liquid crystalline substance.
Applying the phase rule of Gibbs to the phase diagram of Figure 2.1, the maximum number of phases being in equilibrium for this one-component system equals 3 ( = 1 + 2). In this case, the system has zero degrees of freedom. The resulting point in the phase diagram is called a triple point, and it is denoted with a triangle in Figure 2.1. For the unary system shown in Figure 2.1, two triple points are present: the triple point SCG and CIG. At each triple point three two-phase lines intersect, as three different two-two-phase combinations exist. For example, at the triple point SCG the S + C, the C + G and the S + G phase equilibrium lines intersect. The C + G and the S + G equilibrium pressure correspond to the vaporisation curve of the condensed phase and the sublimation curve, respectively, and is equal to the vapour pressure. The S + C equilibrium corresponds to the phase transition of the solid to the condensed phase. For all these two-phase equilibria the equation of Clausius-Clapeyron is valid [43]: coex
dP
H
dT
T V
(2.3) G I C S T P7
Figure 2.2. Schematic representation of a P,T,x–projection of a liquid crystal with a gas. S is the solid phase, C is the liquid crystalline phase, I is the isotropic phase and G is the gas phase. is a triple point of the pure LC, a quadruple point of the mixture. The thin solid lines correspond to the two-phase equilibria of the unary liquid crystal system. The bold lines correspond to the three-phase equilibria of the binary system. The labels next to the lines represent the corresponding phase equilibria.
Equation (2.3) describes that the slope of the phase equilibrium in the P,T-plane is a relation between the enthalpy change ΔH, the temperature T and the volume change ΔV of the phase transition. An illustration of this formula is the S + C phase equilibrium curve, which has a steep slope and a relative small ΔV compared to the sublimation and the C + G curve. Therefore, as an estimation of the triple point temperature, one can use the S to C phase transition temperature at atmospheric pressure.
2.2. Binary systems
Compared to unary systems, binary systems have an additional degree of freedom. Therefore, a P,T-plane is not sufficient anymore to describe the complete phase behaviour of the system. Mostly, the complete phase behaviour
T P SCI CIG SIG SCG x SC CI IG CG SG S C I G
8
is shown using a P,T,x-projection. A schematic example of the binary system of a liquid crystal with a gas is shown in Figure 2.2. First, the P,T-projection of such a system will be discussed, followed by the T,x-projection.
The P,T-projection of Figure 2.2 shows the phase equilibria of the pure liquid crystal as shown in Figure 2.1. These are represented with thin lines. The monovariant three-phase equilibria corresponding to the binary mixture are shown as bold lines. The three-phase equilibria shown are the CIG, SCG, SCI and SIG equilibria. As the binary mixture has one degree of freedom left if there are three phases in equilibrium, these three-phase equilibria are represented as a curve in the P,T-diagram. The SCG and the CIG curve end in the corresponding triple point of the pure component. This is easily understandable if one considers the case of an infinitely small amount of gas added to the liquid crystal: the three-phase equilibrium will be hardly influenced and will be close to the triple point. The initial slope of the three-phase equilibrium curve at the triple point SCG can be calculated using Ipat’evs equation [44]:
* 1 tr LC * tr,LC 1 tr LC tr,LCH
H
dP
dT
T
H
V
RT
(2.4)This equation shows us that the slope of the three-phase curve depends, among others, on the phase transition enthalpy
trH
LC* of the pure liquid crystal.Therefore, the SCG curve is steeper than the CIG curve in the P,T-diagram. Other factors are the volume change of the phase transition of the pure liquid crystal
*tr LC
V
, the temperature of the phase transition of the pure liquid crystalT
tr ,LCandthe Henry coefficient H1. The two aforementioned three-phase curves, SCG and
CIG, intersect in a four-phase point, called a quadruple point. At this point, the four three-phase curves SCG, CIG, SCI and SIG intersect. When the temperature is lower than the temperature of the quadruple point, the nematic phase is not stable anymore. As the number of degrees of freedom at the quadruple point is zero, the four phases have a unique composition and therefore, it is difficult to measure this point. The quadruple point is usually estimated from the intersection of the three-phase curves directly.
With the main elements of the P,T-plane discussed, the T,x-plane will be considered. First, the nature of a three-phase equilibrium at a constant composition is examined. In this case, at a specific temperature and pressure, all the phases are in equilibrium, but the compositions of the different phases are
9
Figure 2.3. P,T,x-projection of a binary system of a liquid crystal with a gas. At (constant) pressure P1, the intersections with the three- and two-phase
curves are visualized in both the P,T- and the T,x-plane. S is the solid phase, C is the liquid crystalline phase, I is the isotropic phase and G is the gas phase. is the triple point of the pure LC, the quadruple point of the mixture.
in principle not equal. Therefore, in the T,x-plane the three-phase equilibrium is represented with three lines, each line describing the composition of one of the phases. For the CIG three-phase equilibrium in a binary system of a liquid crystal and a gas, one can consider the gas-phase as an almost pure gas. In the case of a liquid crystal and a gas, the S phase in the SCG equilibrium can be considered to be pure LC. All the different lines of the three-phase equilibria end in the points of the quadruple point, as the three-phase equilibria in the P,T-plane do. A T,x-phase diagram for a specific pressure, say P1, can be used for designing the
CO2 capture process, as these will show the phase equilibria present and the
phase boundaries at the specified process conditions. The procedure of creating a T,x-phase diagram from the P,T,x-projection is shown in Figure 2.3. At pressure
P1 in the P,T-projection, first the SCG curve is crossed. Second, the SC phase transition of the pure component is crossed, and third the CIG curve and the CI
T P SCI CIG SIG SCG x P1 SCI SIG SC CI IG CG SG S C I G
10
Figure 2.4. T,x-projection of a binary system of a liquid crystal with a gas at (constant) pressure P1 (see Figure 2.3.) S is the solid phase, C is the liquid
crystalline phase, I is the isotropic phase and G is the gas phase. is the triple point of the pure LC. The circles correspond to the intersections with the three-phase equilibria.
phase transition of the pure component. At last, the IG curve of the pure component is crossed. The pure component two-phase equilibria are situated on the T-axes of the T,x-diagram. The compositions of the three-phase equilibria can be found by reading the corresponding curves in the T,x-projection which take part in the equilibrium. The resulting T,x-diagram is shown in Figure 2.4.
T xgas I + G C + G S + G S + C C I G C + I
11
3. Phase behaviour of liquid crystals with CO
2This chapter is based on: M. de Groen, T.J.H. Vlugt, T.W. de Loos, Phase behaviour of liquid crystals with CO2, Journal of Physical Chemistry B, 116 (2012) 9101-9106
3.1. Introduction
For a CO2 capture process using liquid crystals, the gas phase can be separated
from the liquid phase at a three-phase equilibrium CIG: at this point, a structured phase (C), an isotropic (I) and a gas phase (G) coexist. C denotes a condensed phase: a solid phase, a smectic phase or a nematic phase. In Figure 3.1, the absorption-desorption cycle is shown in a T,x-diagram. The difference in solubility between the structured and isotropic phase determines the amount of CO2 that can be captured during an absorption-desorption cycle.
This depends on the width of the two-phase area C + I. The difference between the initial slopes (xCO2 → 0) of the C ↔ C + I and the C + I ↔ I curves depends
on the phase transition enthalpy and can be calculated using a modified van ‘t Hoff law [43]: 2 2 C I CO CO tr 2 tr dx dx H dT dT RT (3.1)
Here ΔtrH is the phase transition enthalpy, which is positive for the structured to
isotropic phase transition of the pure LC, 2 C CO
x
and 2 I COx
are the mole fractions of CO2 in the structured and isotropic phase, respectively, T is the temperature, Ttris the phase transition temperature and R is the gas constant.
For capturing CO2, nematic liquid crystals are most interesting because the
viscosity of nematic LCs is much lower than that of the smectic LCs. Eq. (3.1) can only be used qualitatively as a measure of the width of the two-phase region for nematic liquid crystals, as published values for ΔNIH, the phase transition
enthalpy for the nematic to isotropic phase, strongly scatter. As an example, in Table 3.1 typical literature values for ΔNIH are reported.
12
Table 3.1. Phase transition temperatures and enthalpies for the pure LCs tested. S denotes the solid phase, Sm the smectic, N the nematic and I the isotropic phase.
Molecule TSN / K TSmN / K TNI / K ∆NIH / kJ/mol TSmI / K 5CB 296 [45] 296.9c 308 [46] 308.35 [47] 308.4c 0.54 [46] 0.39 [47] 5OCB 320.5a [46] 325.5b [48] 326.8b,c 340.71 [46] 340.55 [47] 341.5c 0.42 [46] 0.20 [47] 8OCB 326.01 [46] 327.7d 339.80 [46] 352.58 [46] 352.85 [47] 353.2d 0.88 [46] 0.40 [47] 3,4-BCH 370c 2,3-BCH 341c a: Solid phase S
1, b: Solid phase S2, c: this work, d: Provided by Prof. Picken [49].
x (CO2) T 0 1 C+G I+G I C C+I (CIG) ads des x (CO2) T 0 1 C+G I+G I C C+I (CIG) ads des
Figure 3.1. T,x-diagram at constant pressure of a liquid crystal with CO2. C
is a structured liquid phase, I is the isotropic phase and G is the gas phase. The line represents the CO2 adsorption (ads) and desorption (des)
13 Another parameter strongly influencing the applicability of liquid crystals is the solubility of CO2. A few solubility measurements of CO2 in liquid crystals are
described in literature [35-37,50]. Of the liquid crystals measured, PCH5 has the highest mass based CO2 solubility, followed by PCH8-CNS and MBBA [37]. A sharp
increase in solubility between the isotropic and nematic phase was found [31]. The method used in literature was a gravimetric method. The phase behaviour of liquid crystal + CO2 systems has not studied before, only the presence of a
two-phase region was found [35].
In this chapter, experimental P,T-phase diagrams of binary mixtures of CO2 with
different liquid crystals with varying polarity and different alkyl chain length will be discussed at a weight fraction wCO2 = 0.05. These phase diagrams will be used
to discuss the influence of molecular structure and polarity on the solubility of CO2 in liquid crystals.
3.2. Experimental methods
Materials. 4’-pentyloxy-4-cyanobiphenyl, purity 99% mass (5OCB) and
4’-pentyl-4-cyanobiphenyl, purity 99% mass (5CB) were obtained from Alfa Aesar, 4-ethyl-4’-propyl-bicyclohexyl, purity >98% mass (2,3-BCH) and 4-propyl- 4’-butyl-bicyclohexyl, purity >98% mass (3,4-BCH) were kindly supplied by Merck, and 4’-octyloxy-4-cyanobiphenyl, purity >98% mass (8OCB) was kindly supplied by Prof. Picken, Delft University of Technology, Delft, The Netherlands. Carbon dioxide was obtained from Linde Gas, with a purity of 4.5. All chemicals were used as received. In Table 3.1 the pure component properties of these liquid crystals are listed.
Setup used. Phase equilibrium measurements were performed according to the
synthetic visual method using a Cailletet setup, which is described in detail elsewhere [51]. Up to 365 K, the temperature was controlled with a Lauda RC20 thermostatic water bath within 0.02 K. For liquid crystals with a clearing point higher than 360 K, a silicone oil bath was used, controlled with a Shimaden DSM temperature control unit within 0.02 K. The pressure was measured using a Budenberg or a de Wit pressure balance, both with an uncertainty of 0.005 MPa. The temperature was measured with an ASL F250 Pt100 thermometer with an uncertainty of 0.01 K.
Sample Preparation. 0.1–0.15 g of liquid crystal was weighed in a so-called
14
impurities. To prevent evaporation of the liquid crystal, the top of the tube was cooled with liquid nitrogen when vacuum was applied. After completion of the degassing, CO2 was added as a gas from a calibrated volume using a gas dosing
system [51].
Method. The maximum error of isotropic + gas to isotropic (I + G ↔ I), nematic
+ gas to nematic (N ↔ N + G), and nematic + isotropic + gas (NIG) equilibria, measured at constant temperature, is ±0.005 MPa. The maximum error of the solid + nematic to nematic (S + N ↔ N), solid + isotropic to isotropic (S + I ↔ I), smectic + nematic to nematic (Sm + N ↔ N), smectic + isotropic to isotropic (Sm + I ↔ I), solid + nematic + gas (SNG), solid + isotropic + gas (SIG), smectic + nematic + gas (SmNG) and smectic + isotropic + gas (SmIG) equilibria measured at constant pressure, is ± 0.03 K. The nematic + isotropic to isotropic (N + I ↔ I) and the nematic to nematic + isotropic (N ↔ N + I) transitions have been measured within an accuracy of 0.01K, unless otherwise stated. In the case of equilibria involving a smectic phase, only transitions in which the smectic phase disappears could be measured, because of the high viscosity of this phase.
3.3. Results
The liquid crystals tested can be divided in three different classes: apolar liquid crystals, polar liquid crystals and weakly polar liquid crystals. An overview of the measured data points is available in Appendix A. The apolar liquid crystals, 2,3-BCH and 3,4-BCH, have a crystal to smectic and at a higher temperature a smectic to isotropic phase transition. Phase equilibria measured for these systems are the three-phase curve SmIG, the bubble-point curve I + G ↔ I and the curve separating the two-phase region Sm + I and the one-phase area I . The phase diagrams of 2,3-BCH + CO2 and 3,4-BCH + CO2, both with a weight fraction
of CO2 of wCO2 = 0.05 are provided in Figure 3.2 and 3.3. The addition of CO2
causes melting point depression: the smectic to isotropic phase transition temperature is shifted to lower temperatures. This is in agreement with Equation (3.1). For comparison, the pure component Sm ↔ I curve is shown in Figure 3.2 and 3.3 as dashed curve. The I + G ↔ I curve of 3,4-BCH is found at a higher pressure than the I + G ↔ I curve of 2,3-BCH.
The pure liquid crystal 8OCB shows a different behaviour: at atmospheric pressure and 326 K, it has a phase transition from solid to smectic, at 340 K from the smectic to the nematic phase and at 353 K from the nematic to the isotropic phase [46]. The phase diagram of this liquid crystal with CO2 at wCO2 = 0.05, is
15
Figure 3.2. P,T-diagram of 4-propyl-4’-butyl bicyclohexyl + CO2, wCO2 = 0.05.
Description of symbols used: Sm + I ↔ I, I + G ↔ I, Sm + I + G. The dashed curve is the Sm ↔ I phase transition of the pure LC.
Figure 3.3. P,T-diagram of 4-ethyl-4’-propyl bicyclohexyl + CO2, wCO2 = 0.05.
Description of symbols used: Sm + I ↔ I, I + G ↔ I, Sm + I + G. The three-phase curve has notation between parentheses (...). The dashed curve is the Sm ↔ I phase transition of the pure LC.
350 360 370 0 2 4 6 8 10 SmIG T / K P / M Pa Sm + I SmI I + G Sm + G I 320 325 330 335 340 345 350 355 0 2 4 6 8 10 SmI T / K P / M Pa I I + G (SmIG) Sm + G Sm + I
16
Figure 3.4. P,T-diagram of 4’-octyloxy-4-cyanobiphenyl + CO2, wCO2 = 0.05.
Description of symbols used: Sm + N ↔ N, N ↔ N + I, N + I ↔ I, N + G ↔ N, I + G ↔ I, Sm + N + G, N + I + G, + Sm ↔ N for the pure LC at 1 bar, N ↔ I for the pure LC at 1 bar. Three-phase curves have notations between parentheses (...).
Figure 3.5. Part of P,T-diagram of 4’-pentyloxy-4-cyanobiphenyl + CO2,
wCO2 = 0.05. Description of symbols used: S2 + N + I, S2 + N ↔ N,
N ↔ N + I, N + I ↔ I, N + G ↔ N, I + G ↔ I, S2 + N + G,
metastable S2 + N + G, S1 + N + G, N + I + G, + S2 ↔ N for the pure
LC at 1 bar, S1 ↔ N for the pure LC at 1 bar, N ↔ I for the pure LC at
1 bar. Three-phase curves have notations between parentheses (...).
325 330 335 340 345 350 355 0 2 4 6 8 10 I + G Sm + G P / MPa T / K Sm + N N I N + G (SmNG) (NIG) N + I 300 310 320 330 340 0 2 4 6 (S2NI) N + I (NIG) N + G I + G I (S1NG) (S2NG) S + G P / MPa T / K S2 + I (S2NG) N S2 + N metastable .
17 presented in Figure 3.4. Bubble-points were measured for both the nematic and the isotropic phase. Compared to the endpoint of the bubble-point curve of the nematic phase, the bubble-point curve of the isotropic phase starts at a lower pressure, indicating a solubility difference. The temperature of the smectic to nematic and the nematic to isotropic phase transition is shifted to lower temperatures and due to the binary nature of the system the phase transition became a trajectory instead of a sharp transition. The width of the two-phase area N + I is 0.8 K and the isotropic phase starts to form at 332.8 K at 3.9 MPa. Two three-phase curves were measured for this system: the SmNG curve and the NIG curve. The SmNG curve is measured from the intersection of the bubble-point curve and the Sm + N ↔ N phase transition line and the NIG curve starts at the intersection of the bubble-point curve and the N ↔ N + I phase transition line.
The phase diagram of the binary system 5OCB + CO2 at wCO2 = 0.05 is shown in
Figure 3.5. For 5OCB, two solid phases were found, S1 and S2. Pure 5OCB has an
atmospheric N ↔ I transition at 341 K, a S1 ↔ N transition at 324 K and a
metastable S2 ↔ N transition at 317 K according to the results in Chapter 4.
Figure 3.5 shows the phase behaviour of the aforementioned binary system. When CO2 is added to the liquid crystal, the S2 phase becomes more stable than
the S1 phase. Therefore, a quadruple point S1S2NG was found at 314.9 K, 2.49
MPa. At this point, four three-phase curves should intersect: the S1NG, S2NG,
S1S2N and S1S2G curve. The last two mentioned three-phase curves could not be
detected with the used experimental method as it would require the detection of solid-solid transitions. At temperatures lower than this quadruple point, S2 is
most stable and the three-phase equilibrium line S2NG is found. However, as the
S1 ↔ S2 phase transition does not occur instantaneous, part of this line is
measured in the metastable region. As the melting point depression for the nematic phase is larger than for the solid phase, an intersection of the S2NI and
the S2NG line is found at 312.8 K, 3.32 MPa. This intersection is the quadruple
point S2NIG, where the three-phase equilibrium curves S2NG, NIG, S2NI and S2IG
intersect. At temperatures lower than the quadruple point, the nematic phase becomes metastable. The boundary between the S2 + I and S2 + N two-phase
regions is the S2NI three-phase line. At temperatures below this quadruple point
the nematic phase is not stable anymore.
Pure 5CB has a crystal ↔ nematic and a nematic ↔ isotropic phase transition. The phase diagram of this liquid crystal with CO2 is shown in Figure 3.6. The
18
Figure 3.6. Part of P,T-diagram of 4’-pentyl-4-cyanobiphenyl + CO2,
wCO2 = 0.05. Description of symbols used: S + I ↔ I, S + I + G, I + G
↔ I, S + N + G, N + I + G, + S ↔ N for the pure LC at 1 bar, N ↔ I for the pure LC at 1 bar. Three-phase curves have notations between parentheses (...).
Figure 3.7. Metastable part of the P,T-diagram of 4’-pentyl-4-cyano-biphenyl + CO2, wCO2 = 0.05. Description of symbols used: N ↔ N + I,
N + I ↔ I, N + G ↔ N, I + G ↔ I, N + I + G, N ↔ I phase transition for the pure LC at 1 bar. The three-phase curve has notation between parentheses (...). 270 280 290 300 310 320 330 340 0 2 4 (SIG) I P / MPa T / K S + I I + G N + G (NIG) (SNG) 270 280 290 300 310 320 330 340 0 2 4 6 8 10 P / MPa T / K I N N + I N + G I + G (NIG)
19 component is smaller than that of the other liquid crystals tested, the quadruple point SNIG is found at a lower pressure than the I + G ↔ I curve, and a stable N + I ↔ I curve could not be measured. At the quadruple point, which is found around 288 K, 1.4 MPa, the three-phase curves NIG, SNG, SIG intersect. The fourth three-phase curve intersecting at this point would be the SNI phase transition,but this could not be measured at this concentration. When the solid is not crystallizing, the metastable part of the phase diagram is found, with the nematic to isotropic phase transition. The metastable part of the diagram is shown in Figure 3.7.
3.4. Discussion
The solubility of CO2 is influenced by the polarity of the liquid crystal. As
illustrated in Figure 3.8, the apolar liquid crystals have the lowest mass based solubility and 5CB the highest solubility. The solubility of CO2 in the case of 5CB
is higher than in the case of 5OCB, but in literature ether groups are considered to increase the solubility of CO2 [52]. A possible explanation for this behaviour is
a distorted quadrupolar moment of the benzene ring, leading to decreased affinity for CO2. Benzene rings have a quadrupolar moment, and ab initio
computations have shown that CO2 forms transient heterodimers with benzene
[53]. However, as the system is dense, the interaction between CO2 and the
benzene ring will also depend on for example the packing of the molecules and solvent-solvent interactions.
Henry coefficients can be used to rank the CO2 capacity of the solvents. The
Henry coefficient can be calculated using:
2 CO2 2 CO 1 0 CO/ MPa
/ MPa
lim
xf
H
x
(3.2)In equation (3.2), fCO2 is the fugacity of CO2 at the experimentally determined
bubble point pressure, xCO2 is the mole fraction of the sample and H is the Henry
coefficient. Assuming that the gas phase is pure CO2 and the isotropic phase is
an ideal mixture, the Henry coefficient, H1, of CO2 in the isotropic phase can be
estimated using 2 2 1 CO CO f H x (3.3)
20
Figure 3.8. Bubble-point curves of various liquid crystals with wCO2 = 0.05.
4-propyl-4’-butyl bicyclohexyl, 4-ethyl-4’-propyl bicyclohexyl, 4’-octyloxy-4-cyanobiphenyl, 4’-pentyloxy-4-cyanobiphenyl, 4’-pentyl-4-cyanobiphenyl.
Table 3.2. Estimated Henry coefficients (H1) for CO2 in LCs at T = 350 K.
LC molecule H1 / MPa
Mole based Mass based
5CB 14.4 66.3 5OCB 15.3 73.5 8OCB 13.6 73.0 PB 15.5 74.2 EP 15.9 70.1 300 320 340 360 2 3 4 5 P / MPa T / K
21 Experimental bubble-point data from the experiments was used to calculate the Henry coefficients at 350 K using an equation of state for calculating the fugacity coefficient of CO2, see Table 3.2 [54]. In this table it is shown that increasing the
length of the hydrocarbon chain of the LC leads to a higher mole based solubility: 3,4-BCH and 8OCB have a lower mole based Henry coefficient than 2,3-BCH and 5OCB, respectively. For ionic liquids with different chain lengths the same trend is observed [52].
3.5. Conclusion
P,T-phase diagrams have been determined for five binary systems of a liquid
crystal + CO2, at wCO2 = 0.05. Of the liquid crystals investigated, 5CB has the
highest solubility of CO2. The apolar liquid crystals tested, 3,4-BCH and 2,3-BCH,
have the lowest solubility of CO2. These liquid crystals are not suitable for CO2
capture, because of the high viscosity of the smectic phase. 5OCB and 8OCB have a lower solubility of CO2 than 5CB, probably caused by a distortion of the
quadrupole moment of the benzene ring. The molar solubility of CO2 was found
to be higher for molecules with a longer hydrocarbon tail. The liquid crystals tested in this chapter are not promising enough for a CO2 capture process
23
4. Phase behaviour of the system
4’-pentyloxy-4-cyanobiphenyl + CO
2This chapter is based on: M. de Groen, H. Matsuda, T.J.H. Vlugt, T.W. de Loos, Phase behaviour of the system 4'-pentyloxy-4-cyanobiphenyl + CO2, Journal of
Chemical Thermodynamics, 59 (2013) 20-27.
4.1. Introduction
To design a process for CO2 capture with liquid crystals, the P,T,x-phase diagram
of a liquid crystal with CO2 should be known to be able to select the proper
process conditions. Such a study has not been executed before.
In this chapter, the results of a detailed experimental investigaton on the phase behaviour of the system 4’-pentyloxy-4-cyanobiphenyl (5OCB) (1) + CO2 (2) are
presented at varying CO2 concentrations, whereas in the previous chapter only
the isopleth at a concentration of wCO2 = 0.05 was studied. Based on the
experimentally determined P,T-diagrams (isopleths at constant wCO2
concentration) a proposal is made for a P,T-projection of the binary system. The isopleths were studied up to a CO2 mole fraction of 0.72, at a temperature range
of 280 – 360 K, and at pressures up to 12 MPa. For the isotropic phase, the molar Henry coefficient was calculated at 341 K to be 13.7 MPa based on the
f,x-diagram up to x2 = 0.327. This chapter is organized as follows. In section 4.2, the experimental method will be described, in section 4.3 the measured isopleths and in section 4.4 the discussion of these isopleths with a proposal for a P,T-projection.
4.2. Experimental methods
Materials. 4’-pentyloxy-4-cyanobiphenyl, 99 mass % (5OCB) was obtained from
Alfa Aesar and used as received. Carbon dioxide was obtained from Linde Gas, with a purity of 4.5. An overview of the materials used is presented in Table 4.1.
Phase equilibrium measurements. Phase equilibria were visually measured
according to the synthetic method using a Cailletet apparatus, as described in
24
Chemical name Source Purity Purification
CO2 Linde Gas Volume
fraction 0.99995
Used as received
4’-pentyloxy-4-cyanobiphenyl (5OCB)
Alfa Aesar Mass fraction (GC) 0.999
Used as received
Chapter 3. Phase equilibria involving the disappearance of a solid phase were measured by changing the temperature while keeping the pressure constant, with an accuracy of 0.05 K unless stated otherwise. The phase boundaries between the two-phase regions nematic + isotropic and the homogeneous one-phase regions nematic or isotropic were measured at constant pressure with an accuracy of 0.01 K. The other phase equilibria were measured by varying the pressure while the temperature was kept constant, with an accuracy of 0.005 MPa, unless stated otherwise.
4.3. Results
P,T-phase diagrams of pure 5OCB and of 5OCB + CO2 mixtures [(1-xCO2) 5OCB + xCO2 CO2] for mole fractions xCO2 = 0.057, 0.159, 0.241, 0.329, 0.400, 0.497 and
0.720 have been measured. The experimental results are presented in Figures 4.1-4.8. The tables with the measured data are presented in Appendix B. In the binary system, next to two solid modifications of 5OCB (S1 and S2) also two
isotropic liquid phases (I1 and I2) were found.
The pressure dependence of the solid (S1) to nematic (N) and of the nematic (N)
to isotropic (I2) phase transition of pure 5OCB is shown in Figure 4.1. According
to DSC measurements in literature, 5OCB has a nematic to isotropic liquid transition at 340.6 K and two different solid to nematic phase transitions at 320.5 and 325.5 K [46,48,55] at atmospheric pressure. The extrapolated data of the phase transitions to a pressure of 0.1 MPa, gave a solid to nematic phase transition at 326.8 K and a nematic to isotropic phase transition at 341.5 K, which corresponds well with the results in literature [46,48,55], the maximum deviation is 1.3 K. In literature, also a transition of another solid modification (S2)
25
Figure 4.1. P,T-diagram of pure 5OCB. S1 denotes the solid phase, N the
nematic phase and I2 the isotropic phase. Description of the used symbols:
S1 ↔ N, N ↔ I2, S1 ↔ N literature value at 0.1 MPa, N ↔ I
literature value at 0.1 MPa [55].
Figure 4.2. P,T-diagram of the system 5OCB + CO2 for xCO2 = 0.057.
Description of the used symbols: S2 + N ↔ N, N ↔ N + I2, N + I2
↔ I2, N + G ↔ N, I2 + G ↔ I2, S1 ↔ N for the pure LC at 0.1 MPa,
N ↔ I for pure 5OCB at 0.1 MPa. Notations between parentheses (...) denote three-phase curves. Dotted lines are added as a guide to the eye.
300 310 320 330 340 350 0 2 4 6 8 10 I2 N S1 P / M Pa T / K S1 320 330 340 350 0 2 4 6 8 10 12 I 2 N P / M Pa T / K S1 + N I2 + G N + G S1 + G (NI 2G) (S1NG) N + I2 N + I2 I2 N + G N I2+ G
26
Figure 4.3. P,T-diagram of the system 5OCB + CO2 for xCO2 = 0.159.
Description of the used symbols: N ↔ N + I2, N + I2 ↔ I2, N + G ↔
N, I2 + G ↔ I2, N + I2 + G, S1 + N ↔ N, S1 ↔ N for pure 5OCB at
0.1 MPa, N ↔ I2 for pure 5OCB at 0.1 MPa. Notations between
parentheses (...) denote three-phase curves. Dotted lines are added as a guide to the eye.
to nematic is reported [55]. However, this transition was not found in this study for pure 5OCB.
The phase diagram of 5OCB + CO2 at xCO2 = 0.057, shown in Figure 4.2, shows
behaviour which is to be expected for a binary mixture of a liquid crystal with CO2. For this composition, the bubble-points of the nematic phase (N + G ↔ N)
and of the isotropic liquid phase (I2 + G ↔ I2) were measured. The two-phase
region N + I2 is very narrow, the width is only 0.13 K. This two-phase region
separates the homogeneous one-phase region N found at lower temperature from the one-phase region I2 at higher temperature. At low pressure this
two-phase region is separated from the two-two-phase region N + G by the NI2G
three-phase curve (see inset in Figure 4.2). The pressure difference of the points of intersection of the N + I2 ↔ N curve and the N + G ↔ N and that of the N + I2
↔ I2 curve and the I2 + G ↔ I2 curve is only 0.02 MPa. In Figure 4.1 also points
310 320 330 340 0 1 2 3 4 5 I 2 P / M Pa T / K N + G I2 + G S1 + G S 1 + N N (S1NG) (NI2G) N + I 2
27 of the S1 + N ↔ N phase boundary are plotted. The three-phase lines S1NG and
NI2G, are indicated with dotted lines in the Figure 4.2.
The system of 5OCB + CO2 for xCO2 = 0.159 shows the same type of diagram as
the phase diagram of xCO2 = 0.057, see Figure 4.3. Compared to the system 5OCB
+ CO2 for xCO2 = 0.057, apart from the three-phase curve NI2G the same phase
equilibria were measured. The three-phase curve NI2G will end at the pure
component’s triple point NI2G. As the N ↔ I2 curve of pure 5OCB is very steep,
it can be assumed that this triple point practically coincides with the phase transition N ↔ I2 of pure 5OCB at atmospheric pressure. At this higher
concentration the N + I2 two-phase region has become slightly wider and the
phase boundaries of the two-phase region, N + I2 ↔ I2 and S1 + N ↔ N, were
found at lower temperatures than the same phase equilibria of the system with
xCO2 = 0.057. However, the temperature difference between the boundaries of the homogeneous nematic phase, S1 + N ↔ N and N ↔ N + I2, has become
smaller because the slope dP/dT is higher for the S1NG curve than for the NI2G
curve.
The phase diagram of the system 5OCB + CO2 for xCO2 = 0.241, shown in Figure
4.4, has been reported in Chapter 3. At this concentration, two different solid modifications were found, S1 and S2. Two three-phase equilibria of solid 5OCB,
the nematic phase and the gas phase were found: S2NG and S1NG. Part of the
S2NG line was measured in the metastable region. These two three-phase curves
intersect in a quadruple point S1S2NG. The coordinates of this quadruple point
were determined by intersecting the two extrapolated three-phase curves and found to be T = (315.2 ± 0.1) K and P = (2.43 ± 0.01) MPa. The other two three-phase curves intersecting at this quadruple point should be S1S2N and S1S2G, but
these phase equilibria could not be measured, as this would require the detection of a S1 ↔ S2 phase transition. This type of phase transition cannot be
determined visually. The other quadruple point found in this system, S2NI2G,
calculated from the experiments at T = (313.0 ± 0.1) K, P = (3.24 ± 0.01) MPa, is the point where the nematic to isotropic and the solid to nematic phase transition lines coincide: a S2NI2 phase transition line is measured. Other
three-phase curves intersecting at this point should be the S2NG, S2I2G and NI2G curve.
A higher CO2 mole fraction is required to measure the intersection of the last two
three-phase curves. When the three-phase curves S1NG, S2NG and NI2G are
extrapolated to higher temperature, the binary three-phase curves correctly end in the pure component triple points, the metastable S2NG triple point, the S1NG
28
Figure 4.4. Part of P,T-diagram of the system 5OCB + CO2 for xCO2 = 0.241,
reproduced from [56]. Description of the used symbols: S2 + N + I2, S2
+ N ↔ N, N ↔ N + I2, N + I2 ↔ I2, N + G ↔ N, I2 + G ↔ I2,
S2 + N + G, metastable S2+ N + G, S1 + N + G, N + I2 + G, S2 ↔ N
for pure 5OCB at 0.1 MPa, S1 ↔ N for pure 5OCB at 0.1 MPa, N ↔ I
for pure 5OCB at 0.1 MPa. Notations between parentheses (...) denote three-phase curves. Dotted lines are added as a guide to the eye.
300 310 320 330 340 0 2 4 6 (S 2NI2) N + I2 (NI2G) N + G I2 + G I 2 (S1NG) (S2NG) S + G P / M Pa T / K S2 + I2 (S2NG) N S2 + N metastable
29
Figure 4.5. Part of the P,T-diagram of the system 5OCB + CO2 for xCO2 =
0.329. Only the stable phase equilibria are shown. Description of the used symbols: S2 + I2 ↔ I2, I2 + G ↔ I2, S2 + I2 + G, S2 + N + G, N +
I2 + G. Notations between parentheses (...) denote three-phase curves.
Dotted lines are added as a guide to the eye.
Figure 4.6. P,T-diagram of the system 5OCB + CO2 for xCO2 = 0.400.
Description of the used symbols: S2 + I2 ↔ I2, I2 + G ↔ I2,
metastable N + I2 + G, S2 ↔ N for pure 5OCB at 0.1 MPa, N ↔ I for
pure 5OCB at 0.1 MPa. Notations between parentheses (...) denote three-phase curves. Dotted lines are added as a guide to the eye.
310 320 330 340 2 4 6 (S2I2G) P / M Pa T / K I2 S 2 + I2 I2 + G N + G S2 + G (NI2G) (S2NG) 290 300 310 320 330 340 0 2 4 6 8 10 P / M Pa T / K S2 + I2 I 2 I2 + G N + G S2 + G (NI2G) metastable (S 2NG) (S 2I2G) (NI 2G) stable
30
Figure 4.7. P,T-diagram of the system 5OCB + CO2 for xCO2 = 0.497.
Description of the used symbols: S2 + N ↔ N, I2 + G ↔ I2. Notation
between parentheses (...) denotes three-phase curve. Dotted lines are added as a guide to the eye.
Figure 4.8. P,T-diagram of the system 5OCB + CO2 for xCO2 = 0.720.
Description of the used symbols: S2 + I1 + G, S2 + I1 + I2, + I1 + I2 + G,
critical end point of I1I2G curve, S2 + I2 + G. Notations between
parentheses (...) denote three-phase curves.
300 310 320 330 340 350 360 0 2 4 6 8 10 12 P / M Pa T / K I 2 I2 + G S2 + G S2 + I2 (S 2I2G) 270 280 290 300 310 320 330 0 2 4 6 8 10 S 2 + G P / M Pa T / K (S 2I1I2) (I1I2G) (S2I2G) (S2I1G) S 2 + I1 I 1 + I2 I 2 + G
31 again that the transition temperature at atmospheric pressure is equal to the triple-point temperature.
The P,T-diagram of the system 5OCB + CO2 for xCO2 = 0.329 is presented in Figure
4.5. At this concentration, the quadruple point S2NI2G is at lower pressure than
the bubble-point curve I + G ↔ I. Three of the four three-phase curves ending at this quadruple point, S2I2G, S2NG and NI2G could be measured up to this
quadruple point. The phase boundaries of the N + I2 two-phase region, N + I2 ↔
I2 and the N ↔ N + I2 could still be measured, but both are metastable. The width
of the N + I2 two phase region is still increasing with increasing CO2 concentration.
The calculated quadruple point S2NI2G was found at T = (313.0 ± 0.1) K, P = (3.21 ± 0.01) MPa. At xCO2 = 0.400 and xCO2 = 0.497, shown in Figure 4.6 and
Figure 4.7, the phase diagram has the same structure as the phase diagram for
xCO2 = 0.329, but the metastable nematic to isotropic phase transitions could not be measured because the mixture crystallized before this phase equilibrium could be detected. In these figures the non-measured three-phase curves are added as dashed curves.
The phase diagram of xCO2 = 0.720 with 5OCB, presented in Figure 4.8, shows
liquid-liquid demixing. The three-phase curve liquid-liquid gas ends at high temperature in a critical end point I2 + (I1 = G) at T = (304.4 ± 0.1) K and P = (7.41 ± 0.01) MPa, and at lower temperature in a quadruple point S1I1I2G
calculated from the experiments at T = (301.6 ± 0.1) K, P = (6.97 ± 0.03) MPa. At this point the S2I1I2, S2I1G, the S2I2G and the I1I2G three-phase curves intersect.
The S2I1G and the I1I2G curves almost coincide with the vapour pressure curve of
pure CO2, the temperature of the critical end point of the 5OCB + CO2 system is
slightly higher than the critical temperature of pure CO2 [54].
4.4. Discussion
A f,x-diagram at constant temperature was used to calculate the Henry coefficient of CO2 in the isotropic phase from the I2 + G ↔ I2 bubble-point curves.
The Henry coefficient can be calculated using Equation (3.2). The fugacity was calculated using the equation of state for pure carbon dioxide of Span and Wagner [54], assuming that the gas phase consists of pure CO2. A second order
polynomial was used for fitting the bubble-points at 341 K up to xCO2 = 0.329.
Figure 4.9 shows the resulting figure. Up to xCO2 = 0.329 the experimental points
32
Figure 4.9. f,x-diagram of the system 5OCB + CO2 up to xCO2 = 0.329 for the
isotropic phase. The line is a fitted second order polynomial. f is the fugacity of gaseous pure CO2 at bubble-point conditions, calculated using
the equation of state of Span and Wagner [54].
0.04 MPa. The Henry coefficient determined at 341 K by calculating the derivative of this linear function at xCO2 = 0 was found to be 14.0 MPa.
In literature, three pure component phase transitions of 5OCB are reported: a S2 ↔ N phase transition at T = 320 K, a S1 ↔ N phase transition at T = 326 K and
a N ↔ I2 phase transition at 340.6 K [55]. No solid-solid transition has been
reported in literature. A reason could be that these phase transitions can have a low phase transition enthalpy; these are not always detectable with DSC or a similar method.
The S2 ↔ N phase transition was only found when liquid 5OCB is quickly cooled
to 253 K, yielding S2 crystals. Upon heating these crystals the S2N transition is
found [57]. In our experiments we were only able to measure the S1N phase
transition with the Cailletet equipment. From the quadruple point S1S2NG found
in the system 5OCB + CO2 at xCO2 = 0.241 one can conclude that a S1S2N and a
S1S2G three-phase curve should be present in the system. Since CO2 does not
0.0 0.1 0.2 0.3 0.4 0 1 2 3 4 5 f / M Pa xCO2
33
Figure 4.10. P,T-projection of 5OCB + CO2. Thin lines correspond to
equilibria of the unary system CO2 or 5OCB, bold lines to equilibria of the
binary system 5OCB + CO2, dashed lines to metastable phase equilibria. The
horizontal dash-dotted line corresponds to the isobar at pressure P1 (see
Figure 4.11). In this system, liquid-liquid demixing is found near the vapour pressure curve of CO2, where I1 is the CO2-rich isotropic phase and I2 the
5OCB-rich isotropic phase. For 5OCB, two solid phases were found: S1 and
S2, in the inset the phase behaviour of the two substances is shown. Other
phases present are solid CO2 (SCO2), the nematic phase N, and the gas phase
G.
dissolve in the solid phase, the temperature of the S2S1 phase transition will not
be influenced by the presence of CO2. This implies that the pure component S2
↔ S1 phase transition coincides with the binary three-phase curves S1S2N and
S1S2G.
Combining the information obtained from the experimental isopleths of Figures 4.1 to 4.8 a schematic P,T-projection of the binary system 5OCB + CO2 can be
constructed. This is shown in Figure 4.10. For pure 5OCB the two-phase curves
I2=G S2G S1S2G or S1S2 S1S2N or S1S2 I2=G P T P1
34
Table 4.2. Interpolated quadruple points for the system 5OCB + CO2. S1 and
S2 denote the solid phases of 5OCB, N denotes the nematic phase, I2 the
5OCB-rich isotropic phase, I1 the CO2 rich isotropic phase and G the gas
phase. xCO2 T / K P / MPa Quadruple point: S1S2NG xCO2 = 0.241 315.2 ± 0.1 2.43 ± 0.01 Quadruple point: S2NI2G xCO2 = 0.241 313.0 ± 0.1 3.24 ± 0.01 xCO2 = 0.329 313.0 ± 0.1 3.20 ± 0.01
Using all available data of S2I2G, S2NG, SNI and NI2G
three-phase curves
313.02-313.15 3.21-3.24
Quadruple point: S2I1I2G
xCO2 = 0.720 301.6 ± 0.1 6.97 ± 0.03
S1S2G, S1S2N, S1NG, S2NG and NI2G. The triple point S2NG is metastable: it is only
found if S1 is not formed. The part of the phase diagram of pure 5OCB including
the 4 triple points S1S2G, S1S2N, S1NG and S2NG resembles the well-known phase
diagram of sulphur [42,58]. The rhombic phase of sulphur should be replaced by S2, the monoclinic phase by S1 and the liquid sulphur phase by N. In the inset in
Figure 4.10 it is clearly visible that the two solids are enantiotropic. For the system 5OCB + CO2 the aforementioned quadruple points S1S2NG (see inset),
S2NI2G and S2I1I2G are found, with their corresponding three-phase lines. An
overview of the P,T coordinates of these quadruple points is presented in Table 4.2. To complete the phase diagram, at low temperatures the triple point of CO2
a quadruple point SCO2S2I1G should be present with the three-phase curves
SCO2S2G, SCO2I2G, S2I1G and SCO2S2I1. As SCO2 will also undergo melting point
depression, this quadruple point should be lower in temperature than the triple point SCO2I1G.
In Figure 4.11, a schematic T,x-diagram at pressure P1 = 3.0 MPa (see Figure 4.10)
35
Figure 4.11. Schematic T,x-diagram at constant pressure P1 = 3.0 MPa (see
Figure 4.10). S1 and S2 denote the solid phases of 5OCB, SCO2 the solid phase
of CO2, N the nematic phase, I1 the CO2-rich isotropic phase, I2 the
5OCB-rich isotropic phase and G the gas phase. The three-phase equilibria are shown between parentheses (...). At the three-phase equilibrium NI2G, CO2
can be captured: the mixture is cooled down from the isotropic phase and splits in a gas phase and a nematic phase. The mole fraction of CO2 in the
nematic phase is lower than in the gas phase.
mixture only a few degrees, leading to a phase transition from the isotropic to the nematic phase. As the solubility of CO2 is larger in the isotropic phase than in
the nematic phase, at constant pressure a gas phase consisting mainly of CO2 will
be formed. The maximum difference in weight fraction of CO2 between the
isotropic and nematic phase is obtained when the pressure of the quadruple point S2NI2G is chosen as the operating pressure, for in this case the S2NG and
the NI2G three-phase curves coincide in the T,x-diagram. This maximum is only
S2+ SCO2 S2+ I1 I1 SCO2+ I1 I1 + G S2+ G S2+ N S1+N N + G G N I2+ G I2 I2 N + (S1S2N) (S2NG) (S2I1G) (SCO2S2I1) (NI2G)
36
wCO2 = 0.001 for 5OCB. Therefore, we feel that 5OCB is not suited for CO2 capture:
the difference between the isotropic and the nematic phase is too small to capture CO2 effectively.
4.5. Conclusion
The binary system 4’-pentyloxy-4-cyanobiphenyl + CO2 shows liquid-liquid
demixing. It is found that 5OCB has two different solid phases, S1 and S2. CO2 can
be captured at the NI2G three-phase equilibrium, but for this molecule the
difference in weight fraction of CO2 between the isotropic and nematic phase is
37
5. Henry coefficients of selected binary mixtures of a
liquid crystal + CO
2This chapter is based on: M. de Groen, B.C. Ramaker, T.J.H. Vlugt, T.W. de Loos, Phase behaviour of liquid crystal + CO2 mixtures, Journal of Chemical and
Engineering Data, 59 (2014) 1667-1672.
5.1. Introduction
To study the feasibility of a CO2 capture process using liquid crystals as a solvent,
the solubility of CO2 in liquid crystals is of particular importance. In Chapter 3 we
found that weakly polar molecules like 4’-pentyl-4-cyanobiphenyl have the highest CO2 solubility. To have a fair basis of comparison, one can use the Henry
coefficient as a quantitative measure of gas solubility. To obtain the Henry coefficient with sufficient accuracy, solubility measurements at a low CO2
concentration should be performed. According to the detailed study of the phase diagram of 4’-pentyloxy-4-cyanobiphenyl with CO2 up to a mole fraction of xCO2 = 0.7, as described in Chapter 4, a concentration of xCO2 ≤ 0.1 should be in
the Henry regime. This chapter focuses on the solubility of CO2 in liquid crystals
with different polarity at a low concentration of CO2, around xCO2 = 0.06 mol/mol.
The liquid crystals studied in this work are 4-ethyl-4’-propyl bicyclohexyl and 4-propyl-4’-butyl bicyclohexyl, two apolar liquid crystals with a smectic phase, and 4’-heptyloxy-4-cyanobiphenyl, 4,4’-hexyloxy benzylidene aminobenzonitrile and 4’-pentyl-4-cyanobiphenyl, liquid crystals with a nematic phase and polar groups. The influence of oxygen groups, size of the molecule and of the cyanogroup is studied. Based on the experimental results we conclude that apolar molecules have a lower solubility than polar molecules. Furthermore, adding a polar group between two benzene rings significantly lowers the solubility of CO2.
5.2. Experimental methods
Materials used. An overview of the chemicals used is provided in Table 5.1.
4-ethyl-4’-propyl bicyclohexyl and 4-propyl-4’-butyl bicyclohexyl, purity ≥ 98% mass were kindly supplied by Merck KGaA. 4’-heptyloxy-4-cyanobiphenyl, purity 99.9% mass (GC) was obtained from Alfa Aesar, 4’-pentyl-4-cyanobiphenyl,
38
Table 5.1. Overview of chemicals used in this chapter.
Chemical name Source Purity Purification
4-ethyl-4’-propyl bicyclohexyl
Merck ≥ 98% mass Used as
received 4-propyl-4’-butyl
bicyclohexyl
Merck ≥ 98% mass Used as
received
4’-heptyloxy-4-cyanobiphenyl
Alfa Aesar 99.9% mass Used as received
4’-pentyl-4-cyanobiphenyl
Alfa Aesar 99% mass Used as received 4,4’-hexyloxy benzylidene aminobenzonitrile Frinton Laboratories 98% mass Used as received
CO2 Linde Gas Volume fraction
0.99995
Used as received
purity 99% mass was obtained from Alfa Aesar, 4,4’-hexyloxy benzylidene aminobenzonitrile, purity 98% mass was obtained from Frinton Laboratories. Dry carbon dioxide, purity 4.5, was obtained from Linde Gas. All chemicals were used without further purification.
The nature of the impurities of the liquid crystals is unkown. However it is likely that the impurities are very similar compounds as the ones studied in this manuscript and the impact of the impurities on the phase behaviour will be very small. DSC measurements of the solid-nematic transitions of the pure liquid crystals agree within 1.5 K with literature values [59], which is fairly good agreement.
Phase Equilibrium Measurements. Phase equilibrium measurements were
performed according to the synthetic visual method using a Cailletet setup, as described in Chapter 3. Phase equilibria measured were the bubble points of the nematic and isotropic phase, determined at constant temperature with an uncertainty of 0.005 MPa. The monovariant three-phase curves nematic + isotropic + gas (NIG) and smectic + isotropic + gas (SmIG) were also determined
39 at constant temperature with an uncertainty of 0.005 MPa. Furthermore, the two-phase transitions of the nematic + isotropic to isotropic state, the nematic + isotropic to nematic state, the smectic + isotropic to isotropic state and the solid + nematic to nematic state were measured by changing the temperature at constant pressure with an uncertainty of 0.02 K, except for the solid + nematic to nematic equilibrium which was measured with an uncertainty of 0.1 K. Because of high viscosities, bubble points of the smectic phase and phase transitions starting from a homogeneous smectic phase could not be measured.
5.3. Results and discussion
Since the proposed separation process is based on the difference in solubility of CO2 between the anisotropic and isotropic phase transition [31], measurements
were focused on equilibria involving the isotropic, nematic and smectic phase. The tabulated data is presented in Appendix C.
The two apolar liquid crystals, 4-ethyl-4’-propyl bicyclohexyl and 4-propyl- 4’-butyl bicyclohexyl, have only a smectic phase. For the pure substances, the smectic to isotropic phase transition at P = 0.1 MPa is found at T = 341 K and 370 K, respectively. For these liquid crystals the phase transitions smectic + isotropic to isotropic, the bubble-point curve isotropic + gas (G) to isotropic and the three-phase curve SmIG were measured. The results are presented in Figures 5.1 and 5.2. The extrapolation of the three-phase curves to zero pressure intersects with the temperature axis at T = 340 K for 4-ethyl-4’-propyl bicyclohexyl and at T = 369 K for 4-propyl-4’-butyl bicyclohexyl. This point of intersection should coincide with the corresponding triple point of the liquid crystal, which can be approximated by the pure component phase transition temperature at atmospheric pressure.
4,4’-Hexyloxy benzylidene aminobenzonitrile has a solid to nematic phase transition at 330 K and a nematic to isotropic phase transition at 375 K at 0.1 MPa [59]. Binary data of 4,4’-hexyloxy benzylidene aminobenzonitrile with CO2 have not been published in literature before. The mole fraction of CO2 in the
mixture was xCO2 = 0.066. Phase equilibria measured for this mixture were the
two-phase to homogeneous phase transitions solid + nematic to nematic, nematic + isotropic to nematic and nematic + isotropic to isotropic. Furthermore, the bubble points of the nematic + gas to nematic and isotropic + gas to isotropic state were measured. The extrapolation of the three-phase curve nematic +
40
Figure 5.1. P,T-diagram of 4-ethyl-4’-propyl bicyclohexyl + CO2, xCO2 = 0.052.
Description of symbols used: Sm + I ↔ I, I + ↔ I, Sm + I + G. The dashed curve is an extrapolation of the three-phase curve Sm + I + G to lower temperatures. The lines are added as a guide to the eye.
Figure 5.2. P,T-diagram of 4-propyl-4’-butylbicyclohexyl + CO2, xCO2 = 0.057.
Description of symbols used: Sm + I ↔ I, I + G ↔ I, Sm + I + G. The dashed curve is an extrapolation of the three-phase curve Sm + I + G to lower temperatures. The lines are added as a guide to the eye.
340 350 360 370 0 2 4 6 8 10 Sm + G Sm + I I + G P / MP a T / K I 360 380 400 420 0 2 4 6 8 10 Sm + G I + G I P / MP a T / K Sm + I
41
Figure 5.3. P,T-diagram of 4,4’-hexyloxy benzylidene aminobenzonitrile + CO2, xCO2 = 0.066. Description of symbols used: S + N ↔ N, N ↔ N +
I, N + I ↔ I, N + G ↔ N, I + G ↔ I, N + I + G. The dashed line is the expected three-phase curve S + N + G. The lines are added as a guide to the eye.
Figure 5.4. P,T-diagram of 4’-heptyloxy-4-cyanobiphenyl + CO2, xCO2 =
0.063. Description of symbols used: N ↔ N + I, N + I ↔ I, N + G ↔ N, I + G ↔ I, N + I + G. The lines are added as a guide to the eye.
320 340 360 380 400 420 0 2 4 6 8 S + G S + N N + G I + G I P / MP a T / K N N + I 320 330 340 350 360 370 0 2 4 6 8 I + G N + G N P / MP a T / K I N + I G r a y
